We studied morphological changes at the maxillary first molar in a model rat for type 2 spontaneous diabetes mellitus (DM), the Goto-Kazizaki (GK) rat, vs. the normal 8-week-old Wistar rat. Serial frontal sections of the gingiva of the maxilla with the bone were prepared from the rats. Image analyses, performed on light micrographs of the hematoxylin-eosin stained specimens, allowed comparison of the thickness of the keratinized, granular, prickle, and basal layers. In addition, the cell population of the granular and prickle layers and the cross-sectional area of the connective tissue beneath the mucosal epithelium were examined. The thickness of the capillary of the maxillary first molar was determined by image analysis of scanning electron micrographs of microvascular corrosion cast specimens. We found that the thickness of the keratinized, granular, and prickle layers was significantly higher in the DM vs. normal group, as were the cell population of the granular and prickle layers. In contrast, the cross-sectional area of the connective tissue beneath the mucosal epithelium, and the thickness of the capillary were significantly lower in the DM vs. normal sections. Therefore, we consider that the DM-associated hyperglycemia causes hypertrophy of the mucosal epithelium, atrophy of the connective tissue beneath the mucosal epithelium, and microangiopathy of the capillary of the palatal gingiva of the maxillary first molar in the GK rat.